Minimizing the positive sodium void coefficient in liquid metal-cooled fast reactor systems



Ms ET Al. 3,367,837 SODIUM VOID C FICIENT IN AST REACTOR STEMS 2 Sheets-Sheet l Feb. 6, 1968 1 B,

MINIMIZING THE PosIT LIQUID-METAL-COOLED F Filed Oct. 24, 1965 m mrnh A. ULI

.m2 hiv Y JmDmlOP D02 /wm Eo J. B. NIMS ETAL Feb. 6, 1968 3,367,837 GIEN -cooLED FAST REAcToR SYSTEMS MINIMIZING THE POSITIVE SODIUM VOID COEFFI LIQUID METAL Filed OCL. 24, 1955 2 Sheets-Sheet 2 .znom mrow NImN CURVE 2 Y CURVE NO. 2-25 FUEL REPLACED BY ZRH2 CURVE NO. 3- 50 FUEL REPLACED BY ZRHZ CURVE CURVE 1 DISTANCE FROM CORE CENTER, CM.

United States Patent O 3,367,837 MHNIMIZING THE POSETVE SODTUM VGH) Cf?- EFFICIENT IN LIQUID METALCQOLED FAST REACTUR SYSTEMS John B. Nima, Royal Oak, and Earl M. Page, Madison Heights, Mich., assignors to Atomic Power Development Associates, Inc., Detroit, Mich., a corporation of New York Filed Oct. 24, 1965, Ser. No. 504,432 Claims. (Cl. 176-18) This invention relates to sodium-cooled or other liquid metal-cooled fast reactor systems and more particularly to the problem of eliminating or minimizing the so-called positive sodium void coefficient which tends to occur with large reactors of that type.

Upon the occurrence of voiding or losses of sodium in the cores of such reactors due to any cause, the result is first a reduction in the moderation effect of the sodium which causes an increase in the neutron energy and an increased reactivity, tending to establish a so-called positive sodium void coefficient, and secondly, the voiding tends to increase leakage of neutrons from the core regions thereby tending to decrease reactivity and to give rise to a negative void coefficient. But such leakage is essentially an effect which is most pronounced in the outer surface or edges of the core system and is not so significant as to voids which occur in the middle zones of the core. Thus for small reactors, the leakage effect outweighs the effects due to reducing moderation so that the positive sodium void coeflicient problem does not ordinarily occur substantially with such small reactors. But for large reactors conversely, the reduction of the Vmoderation effect may outweigh the leakage effect. Thus, assuming that a small disturbance or accident occurs in a large sodium-cooled reactor, giving rise to voids or greater voids, then if any substantial tendency toward increased reactivity results, there will be an increase in power generated which results in the voiding problem becoming worse. Although the aforesaid effect of causing reduction in moderation essentially may occur over the whole core, it is much more pronounced in the central regions.

Ordinarily it is preferable that there be a slightly negative sodium void coefficient in such fast reactors, although certain factors may so alter the situation that a somewhat positive coeliicient may be tolerated with safety.

The increase in reactivity upon loss of sodium (positive sodium void coefficient) large advanced sodium-cooled fast reactor systems, presents a problem to the designers of such systems in their effort to select a core configuration which will be satisfactory as to its safety implications. Thus it has been proposed to circumvent this problem with large reactors by selecting a core geometry which will permit high neutron leakage which tends to occur, for example, if the core is shaped like a fiat cylinder or more or less disclike, or las an annulus, or if a modular core is used containing several loosely coupled smaller cores of more conventional shape. But such designs inherently possess the disadvantages of a low in-core `breeding ratio and high critical mass. The penalties attached to these disadvantages are that the low internal breeding ratio leads to the necessity of a more complex and expensive control system in order to provide a greater amount of reactivity needed for burnup and the larger fuel inventory adds appreciably to the fuel cycle costs. For these reasons, there has been a need for a sol-ution of this positive sodium void coefficient problem which will permit the use of conventional core geometry.

In arriving at an alternative solution of this problem, we have borne in mind that the void coefficient is positive only in the central parts or regions of the core Where the neutron leakage component is generally insignificant, such which is characteristic of leakage component being negative and dominant in the regions near the core boundaries. Thus, according to one aspect of the present invention, we have made use of the possibility of introducing a local modification of the core composition at the central regions thereof rather than one that extends over the entire core region. That is, the desired result is obtained by introducing a fixed moderator material in the central regions of the core and the invention makes use of the variation of neutron importance with energy, and the energy distribution of the neutron flux. It is characteristic of advanced fast reactors that the slowing down of neutrons in the energy range above approximately l() kev. produces a negative reactivity change, while slowing down at lower energys causes a positive change. Depending upon the energy distribution of the neutron fiux, neutron slowing down by sodium can produce a net local reactivity effect that is either positive or negative. In the usual fast reactor designs, the fiux comes to a peak at an energy level at about 200 kev. and thus the moderating component of a sodium void effect tends to be positive. Thus, we have conceived that the void effect can be made more negative by shifting the fiux spectrum toward the low energy range and in the application of this technique, this fiux shift in accordance with the invention is accomplished by introducing a solid moderator material in the central regions of the core as above stated.

In addition to the effect described above, this additional moderation would also tend to reduce the absolute magnitude of the sodium void effect whether it be positive or negative due to the fact that the voidingl of sodium would have a less significant effect on the moderating properties of the system if the sodium constitutes a smaller part of the total moderator present.

Further aspects, Afeatures and advantages of the invention will more clearly appear from the following description taken in connection with the accompanying drawings.

In the drawings.

FIG. 1 is a schematic diagram of a reactor embodying the invention and showing various typical parts and regions as in vertical cross-section;

FIG. 2 is a diagrammatic view with the upper and lower end portions broken away of a typical fuel sub-assembly such as may be used in the central core region in accordance with the invention; and

FIG. 3 comprises certain graphs illustrating the effect on the sodium void coefiicient of the zoned addition if moderating material in the form for example of zirconium hydride.

Referring now to FIG. 1 in further detail, it will be noted that there is here shown a reactor vessel having a normal core region and which has a compact form of geometry. Within the central or middle portions of this core region, there is indicated in different cross hatching a moderated core zone in accordance with the invention. The core region around its sides and above and below is surrounded by a blanket `and reflector region of a suitable type conventional for this class of reactor. A typical one of the normal fuel sub-assemblies is here indicated as extending from the top of the blanket region down through the normal core region and through the lower blanket region to suitable supporting plates. It will be understood that this is merely a schematic showing of one of these typical normal fuel sub-assemblies of which there would be a large number in the complete reactor. Also, there is here shown a typical one of the moderator-fuel sub-assemblies extending from the top down to the supporting plates in the reactor and it will be understood that there will be a considerable number of the sub-assemblies of this type eX- tending down through the moderator core zone. The

normal fuel sub-assemblies may be of suitable known conventional types.

volume in which the void coefficient is positive, up to such a case was reduced from a maximum positive effect of some $2.6 on the base case to only 11 cents in the exam- FIG. 2 shows in further detail that portion of a typical one of the moderator fuel sub-assemblies which extends down through the normal core region as well as the modple where 50% of the fuel had been replaced by modererated middle core region contained therein. That is, such ator. As will be noted in FIG. 3, the local sodium Void sub-assembly will be contained in a can-like wall the sarne coefficient is made significantly more negative over the as are the typical normal fuel sub-assemblies and such entire inner zone although in the more central regions a sub-assembly will contain a number of the so-called norsmall positive worth still exists but not to an ordinarily mal type of fuel pins of which a typical one is shown and prohibitive extent, and in the remaining portions of the also a number of moderator-fuel pins of which a typical middle regions, the coefficient becomes slightly negative. one is also shown and which at its mid portions contains l0 A further advantage of the invention is that of providing a body 0f mOdera'Or material aS SllOWn Separated at its savings in the fuel inventory over and above that achieved upper and lower ends by suitable insulation from the fuel in a low leakage geometry for the purpose. However, the contained in the uppermost and lowermost portions of softer spectrum near the core center' produces the disadsuch fuel pin. vantage of a larger power peaking factor and a reduced It has been heretofore determined that in the usual case 15 breeding ratio. Thus the overall merits of this zoned specthe positive coefficient zone in such large reactors at least trum solution of the problem can be appreciated more with a compact type of core bodies such as here shown is readily from the following results obtained yfrom investiconfined substantially to the central one-third of the core gation of the 4,000 liter core. The following analysis in- (as measured across the core in any direction). In the cludes the effect of adjusting the inner zone enrichment to Vicinity 0f the boundary 0f this Central POTOU, the VOid 20 reduce the undesirable power peaking and to minimize coeflicient is very small and then becomes negative in the the reduction in breeding ratio. The following Table I outer regions of the core. The fact that the positive zone indicates the results.

TABLE I.-EFFECT OF FUEL REPLACEMENT BY ZrH IN CENTRAL PERCENT OF CORE VOLUME occupies only a fraction of the core is important for the Thus replacing 121/2% of the fuel with moderator in reason that it is preferable to limit the soft spectrum region the 4,000 liter core (using uniform enrichment) reduced in accordance with the invention to the minimum volume. the maximum positive and total core void coefficients It is in this inner zone that part of the fuel be replaced from $4.0 and $3.0 to $1.9 and $0.4 respectively (the by moderator material. The moderator may be 4mixed with 45 maximum positive effect is obtained by integrating the the fuel or preferably placed in separate pins as indicated void effect over only the positive zone). The internal and in FIG. 2, depending on the compatibility of the two matotal breeding ratios reduce from about 1.0 and about terials. In the analyses described herein, ZrHz has been 1.5 to about 0.8 and about 1.2. There is an improvement used as the moderator material and to prevent its decomin critical mass from 1306 kg. to'about 1213 kg. The position, it is located in portions of separate pins. maximum to average power density ratio increases rather However, the invention is not necessarily limited to the markedly from about 2.2 to 2.9.

use of moderating material comprising zirconium hydride The power distribution deterioration and internal breedas it is believed that beryllium oxide will yield similar ing loss may be restored by reducing the inner zone fuel results although it will require about twice the volume enrichment. These effects are indicated in another exfraction of BeO to achieve a given effect. Also yttrium 40 ample of the following Table II in which the central hydride may possibly be used. zone fuel enrichment is 85% of that in the outer zone.

TABLE IL EFFECT OF ZONED ENRICHMENT WITH 12.5 (VOL. PERCENT) 0F FUEL REPLACED BY ZrHg IN CENTRAL 25% OF CORE VOLUME The method of the present invention has been investigated using spherical geometry, for 2,000 and 4,000 liter fast oxide cores. In the central 25% of the total core The power density ratio is essentially that of the unmoderated core and the breeding ratios, both in-core and total, are partly restored. The improvement in the void coefficients is reduced, but the major part of the improvement some of the fuel was replaced with moderator mate- 65 achieved by the spectral softening is retained. rial in the form of ZrH2, the fuel enrichment was uniformly In assessing the accomplishments of the objectives of adjusted to maintain a critical system and the sodium this invention, it should be observed that, based on earlier worth was calculated by means of perturbation theory. studies of 1000 mwe. cores, use of high leakage core The results are shown by the curves of FIG. 3 which indigeometry to gain similar improvements in the void cocate the effect of the zoned ZrH2 addition, on the sodium 70 efficient resulted in critical mass penalties of 30% or more, void coefficientina2,000 liter oxide core. and internal breeding ratios in the range of 0.5 to 0.6. A measure of the possibilities of this technique is evi- The comparison is therefore being made with these values dent from the fact that upon integrating the void coefirather than with the base case numbers which refer t0 cient over all the core regions where the coefficient was a core design unmodied for improvement in the void positive with this core, the sodium void coefficient in coefficient. On this, basis it appearsA clear that the zoned spectrum core approach gives distinct improvements and advantages over the use of the high leakage core approach involving cores of modified geometry. Another aspect relating to safety that is obviously affected is the Doppler coefficient. Without considering this in detail, it is contemplated that its magnitude will be increased substantially in the central zone as a result of the spectrum softening and therefore improve the inherent reactor response in accident situations.

While the invention has lbeen disclosed specically as applied to sodium-cooled reactors, various aspects of the invention may be applicable to other types of liquidcooled reactors and the specification and appended claims should be interpreted in the light of that possibility.

Although certain examples of the invention are herein disclosed for purposes of explanation, various further modifications after study of this specification will be apparent to those skilled in this art and reference should accordingly be had to the appended claims in determining the scope of the invention.

What is claimed and desired to be secured by Letters Patent is:

1. A large liquid-metal cooled fast breeding reactor having a core comprising a transverse and longitudinally defined middle region and an outer region, said outer region having a plurality of fissile fuel elements, the total of the fissile fuel elements being sucient for sustaining a fast nuclear chain reaction, and said middle region having a substantial proportion of spaced moderator elements for shifting the iiux spectrum in said core towards the low energy range, said outer region containing substantially no moderator, whereby the void coefficient for said middle region has only slightly positive or negative values.

2. A liquid-metal cooled fast nuclear reactor according to claim 1 wherein said middle region comprises about one-third of the core volume.

3. A liquid-metal cooled fast nuclear reactor according to claim 1 wherein said moderator elements comprise up to about of the fuel volume of the middle region.

t. A liquid-metal cooled fast nuclear reactor according to claim 1 wherein said moderator elements are fabricated from zirconium hydride'.

5. A liquid-metal cooled fast nuclear reactor according to claim 1 wherein said fuel elements comprise su'bassemblies each containing a plurality of fuel pins, said moderator elements comprising a mid portion of a plurality of pins which extend into the middle region of the core and which contain moderator material.

References Cited OTHER REFERENCES Proceedings of the Second United States Nations International Conference on the Peaceful Uses of Atomic Energy, vol. 12 (1958), pp. 28-31 (Loewenstein et al.) p. 102 (Smith et al.) p. 121 (Lung et aL).

REUBEN EPSTEIN, Primary Examiner. 

1. A LARGE LIQUID-METAL COOLED FAST BREEDING REACTOR HAVING A CORE COMPRISING A TRANSVERSE AND LONGITUDINALLY DEFINED MIDDLE REGION AND AN OUTER REGION, SAID OUTER REGION HAVING A PLLURALITY OF FISSILE FUEL ELEMENTS, THE TOTAL OF THE FISSILE FUEL ELEMENTS BEING SUFFICIENT FOR SUSTAINING A FAST NUCLEAR CHAIN REACTION, AND SAID MIDDLE REGION HAVEING A SUBSTANTIAL PROPORTION OF SPACED MODERATOR ELEMENTS FOR SHIFTING THE FLUX SPECTRUM IN KSAID CORE TOWARDS THE LOW ENERGY RANGE, SAID OUTER REGION CONTAINING SUBSTANTIALLY NO MODERATOR, WHEREBY THE VOID COEFFICIENT FOR SAID MIDDLE REGION HAS ONLY SLIGHTLY POSITIVE OR NEGATIVE VALUES. 